This research proposal is concerned with the synthesis, structures, spectroscopic and reactivity properties of specific iron and molybdenum complexes. New Fe/S clusters will be synthesized, that are expected to serve as synthetic analogues of the Fe3S3 or Fe3S4 sites in certain 3Fe proteins such as aconitase, D. gigas FdII, A, vinelandii FdI, etc. These new Fe/S clusters will be obtained from the [Fe6S6L6]2-,3- clusters recently discovered and characterized in our laboratories. The latter will be completely characterized with various terminal ligands (L) and their structures will be determined by X-ray crystallography. One of the aims of this proposal is to synthesize Fe/Mo/S aggregates with a Fe:Mo:S ratio approaching 6:1:8 or 9. This ratio is presently accepted as possible ratio for the Fe/Mo/S site in nitrogenase. A successful analog for the nitrogenase active site should allow for an understanding of the catalytic conversion (reduction) of dinitrogen to ammonia in biological systems. This reaction also is potentially useful for our understanding of small molecule activation by catalytic metal containing sites in general. Another specific goal of this proposal is the synthesis and complete characterization of mixed-terminal ligand ferredoxin active site analog complexes such as [Fe4S4LnLprime4-n]2-,3- and [Fe2S2LnLprime4-n]-2,-3 with L = RS-, RO- and other uninegative monodentate ligands and L' - bidentate or tridentase chelate ligands with 0 or N donor atoms. These clusters are expected to serve as models for the P-clusters in nitrogenase and the citrate bound Fe4S4 cluster in aconitase. Attempts to stabilize [Fe4S4]3+ cores will be made by replacing 1 or 2 of the biological type ligands (PhS-, Ph0-) from known synthetic analogues with the "organometallic" ligand C5H5- and derivatives. These new mixed terminal ligand clusters are expected to undergo two-electron redox reactions and as such may be important for the reduction of small substrate molecules.